In the past, telecommunication services integrated in an automobile were limited to a few systems, mainly analog radio reception (AM/FM bands), for which a simple whip antenna was mounted to and extended from a vehicle body. A disadvantage of this fixed mast monopole antenna is that it protrudes from the exterior of the vehicle as an unsightly vertical wire with a height of roughly one quarter wavelength of the signal frequency. This is because the whip antenna must exhibit certain mechanical characteristics to achieve user needs and meet required electrical performance. The antenna length, or the length of each element of an antenna array, depends on the received and transmitted signal frequencies. A further disadvantage of the monopole antenna is that it is susceptible to damage due to vandalism and car wash systems.
Further, the monopole antenna has a nearly omnidirectional radiation pattern, which provides a signal sent with approximately the same strength in all directions in a generally horizontal plane, producing a null only towards the sky. Another disadvantage of the monopole antenna is that it is typically narrowband with a bandwidth of roughly ten percent. With the rising number of communication systems, there are a continuously rising number of services that are to be integrated in the vehicle and which require further antennas to be arranged in the vehicle. Further, if antenna diversity is used to provide directional sensitivity, a number of antennas are required. However, since vehicle design is often dictated by styling, the presence of numerous protruding antennas is not desirable.
In an effort to minimize any aesthetically displeasing appearance or visually obstructive antenna characteristics, a trend emerged to embed the antenna system into the vehicle structure, such as, for example, into a rear window. Further, an integration of several telecommunication services into a single antenna is attractive to reduce manufacturing and installation costs of multiple antennas. However, rear window antennas exhibit troubles, for example, pattern disconnection of the thin window antenna often occur.
Not only are the electrical, mechanical and aesthetic properties of an antenna important, but an antenna must also overcome unique performance issues in the wireless environment. Further, antenna integration is becoming more necessary due to a cultural change towards an information society. The Internet has evoked an information age in which people around the globe expect, demand, and receive information. Car drivers expect to be able to drive safely while handling e-mail and telephone calls and obtaining directions, schedules, and other information accessible on the World Wide Web. Telematic devices can be used to automatically notify authorities of an accident and guide rescuers to the car, track stolen vehicles, provide navigation assistance to drivers, call emergency roadside assistance, and provide remote engine diagnostics. In designing the antenna, careful consideration must be given to the antenna electrical characteristics so that signals transmitted from and received by a communications device satisfies pre-determined operational limits, such as the bit error rate, signal-to-noise ratio or signal-to-noise-plus-interference ratio. In a number of applications, an omnidirectional antenna is less effective in achieving optimum values for these characteristics, as compared with a directional antenna.
The directional antenna, provides a concentrated signal or beam in a selected direction. Concentrating the beam increases the antenna gain and directivity. Directional antennas are often utilized to communicate with terrestrial support, with short range communication systems (SRC). Radio frequency (RF) communication signals are typically employed for their advantages of penetrating and passing through objects, their low power, and low cost.
However, directional antennas currently suffer from disadvantages of having complex shapes and large size, making them difficult to package in a vehicle. It is preferable to conceal the antenna to protect it from the environment and to preserve vehicle aesthetics. In order to conceal the antenna, it is usually necessary to locate the antenna beneath the sheet metal body of a vehicle. However, the sheet metal shields and adversely affects the performance of the directional antenna.
A novel directional antenna is described by U.S. patent application Ser. No. 11/987,786, entitled Directional Antenna, assigned to Delphi. In one description, the antenna includes an information signal impressed across a light filament of a vehicle headlight, and a reflective surface directs the electromagnetic radio waves in a predetermined direction. The antenna is fully concealed and can operate using an unmodified, factory installed vehicle headlight. However, the beam pattern is set in part by the physical characteristics and positioning of the filament and the reflective surface.